1. Field of the Invention
The present invention relates to the field of curable and cured perfluoroelastomer (FFKM) compositions and components made therefrom such as sealing materials, which compositions are suitable for various high temperature end applications, particularly for use in the semiconductor area in chemical vapor deposition (CVD) and other semiconductor processing technologies. The present invention provides curable fluorine-containing elastomeric compositions, cured fluorine-containing elastomeric compositions and related molded articles formed from such compositions which are clean compounds and have excellent thermal properties.
2. Description of Related Art
Fluorine-containing elastomers, particularly perfluoroelastomers (FFKM) that include tetrafluoroethylene (TFE) and other fluorinated monomer units exhibit excellent chemical resistance, solvent resistance and heat resistance, and therefore are widely used for a sealing and other materials which are intended for use in harsh environments. As technology advances, the characteristics required even for such highly resistant compounds continue to be more rigorous. In the fields of aeronautics, aerospace, semiconductor and chemical and pharmaceutical manufacturing, sealing properties under harsh chemical environments that are also subject to high temperature environments of not less than 300° C. are encountered, and the ability of such materials to withstand high temperature environments has become increasingly important.
Perfluoroelastomeric materials are also known for their chemical and plasma resistance. However, when used in compositions requiring acceptable compression set resistance and mechanical properties generally include filler or reinforcing systems. It is the goal to use such good high temperature and environmentally resistant materials to form molded parts that are capable of withstanding deformation and that hold up in such rigorous conditions. FFKM materials are typically prepared from perfluorinated monomers, including at least one perfluorinated cure site monomer having a functional group with a cure site. The monomers are polymerized to form a curable perfluorinated polymer having the cure sites thereon which are intended for cross-linking upon reaction with a curative or curing agent. Upon curing (cross-linking), the materials form an elastomeric material. Typical FFKM compositions include a polymerized perfluoropolymer as noted above, a curing agent that will react with the reactive cure site group on the cure site monomer, and any desired fillers. The resulting cured perfluoroelastomeric material exhibits elastomeric characteristics.
FFKMs are also generally known for use as O-rings and in related sealing parts for high-end sealing applications due to their high degree of purity, heat-resistance, and resistance to plasma, chemicals and other harsh environments. There continues to be development of new perfluoroelastomeric compositions due to ever-increasing demands and challenges to be met by FFKMs and compositions based on FFKMs to provide ever higher levels of thermal, chemical and plasma resistance. Industry demands, particularly in the semiconductor area, continue to require enhanced performance of such seals to meet new end-use applications that have increasingly aggressive environments as well as lower and lower contamination and particulation requirements. Thus, there is always the need for better properties but from “clean” compounds, i.e., those that introduce little or no harmful contaminants into the end use environment.
As is recognized in the art, different FFKM compositions may include different curatives (also known as curing agents) depending on the type of cure site monomer (CSM) structure being used and the corresponding curing chemistry applied to react the functional active cure sites on the cure site monomers with the curatives. Such compositions may also include a variety of fillers and combinations of fillers to achieve target mechanical properties, compression set or improved chemical and plasma resistance.
For semiconductor sealing applications, both inorganic and organic fillers have been used to improve plasma resistance depending on the type of plasma chemistry. Typical fillers known in the semiconductor and other industries include carbon black, silica, alumina, TFE-based fluoroplastics, barium sulfate and other polymers and plastics. Fillers used in some FFKM compositions for semiconductor applications include fluoroplastic filler particles formed of polytetrafluoroethylene (PTFE) or melt-processible perfluorinated copolymers such as copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (also referred to as FEP-type copolymers) or of TFE and perfluoroalkylvinyl ethers (PAVEs) (known as PFA-type copolymers), particularly in micro- or nanometer-sized particles.
Such FFKM compositions can include only a single FFKM curable polymer, or sometimes blends of one or more FFKM curable polymers. Similarly there are FFKMs that have only a single cure site on a cure site monomer in the curable perfluoropolymer used in the composition, and FFKMs that have more than one cure site monomer having the same or different cure sites. As demand for cleaner and more chemically resistant compounds increases, the need to achieve properties in FFKMs with little or no filler becomes more of a pressing need. Thus, there are many potential combinations of materials that may be used, and the challenge is achieving higher thermal, chemical and plasma resistant property requirements for various end applications without sacrificing mechanical and sealing properties and preferably also achieving good curing characteristics.
FFKM compositions including semicrystalline fluoroplastic particle fillers, such as microparticles or nanoparticles of PTFE or copolymers such as those of the PFA-type, provide good physical properties, plasma resistance and excellent purity. For semiconductor applications, such systems also help to avoid metallic particulation and contamination at a level improved over FFKMs, which have inorganic fillers such as metal oxides. However, there is a need in the art to develop more simplified processing methods to form fluoropolymer-filled FFKMs. There are various blending processes which have been developed to incorporate TFE-based fluoroplastics into FFKMs to achieve clean compounds. Latex blending has been used but can be expensive for large-scale, commercial batches. Melt blending is also available and generally requires temperatures of up to 350° C. Further, filler loading in many commercial products is generally limited to up to about 30 weight percent of the weight of the base polymeric materials. Use of fluoropolymeric fillers in such compositions can also sometimes contribute to relatively high compression set particularly in end applications at higher temperatures (e.g., >300° C.). Moldability and bondability can also be limited due to use of such fluoropolymeric fillers.
Examples of compounds using such fillers include U.S. Pat. No. 6,710,132 discloses a blend of an FFKM having semi-crystalline fluoroplastic particles (such as PTFE), wherein the particles have a core-shell structure and are formed by latex blending of these materials.
U.S. Pat. No. 4,713,418 discloses a composition formed by melt blending an FFKM and a melt-processible thermoplastic fluoropolymer. The patent asserts that particles of about 10 microns are reformed from some of the melted thermoplastic upon recrystallization. Melt blending is also used in U.S. Pat. No. 7,476,711 to form cured articles from compositions having a blend of FFKM and semi-crystalline thermoplastic copolymer of TFE-PAVE (PFA melt-processible copolymer) present in particles of greater than 100 nm.
Additional fluoroplastic/FFKM blends are disclosed in U.S. Pat. No. 7,019,083 which includes latex particles of a fluoroplastic including a nitrogen-containing cure site which may be combined with an uncured perfluoropolymer (FFKM gum). The latex particles may be of a core-shell structure in which the nitrogen-containing cure sites are provided on the shell of the core-shell particles. The fluoroplastic particles remain in the cured fluoroelastomer matrix upon curing.
U.S. Pat. No. 7,354,974, noted above, discloses melt blending an FFKM and a semicrystalline polymer such as PTFE and/or a copolymer, such as the PFA-type copolymer, of greater than an average size of 100 nm wherein blending temperature or curing temperature exceeds the melting temperature of the fluoroplastic fillers.
Various polymers have also been developed with unique cure systems to provide base FFKM compounds that have improved heat characteristics. For example, U.S. Pat. No. 6,855,774 teaches use of a curing agent which has at least two crosslinkable functional groups represented by the formula:
The curative is proposed for use with FFKM materials having a cure site that is a nitrile group, carboxyl group or alkoxycarbonyl group. The cross-links formed are described as contributing to increased heat resistance. U.S. Pat. No. 6,878,778 further teaches curatives having at least two groups having the formula:
wherein R1 in each of the groups may be a fluorine atom or a monovalent organic group. Resulting polymers formed using the curatives are described as having excellent chemical resistance and mechanical strength as well as heat resistance at high temperatures.
Blended FFKMs have also been developed to achieve unique properties. FFKMs such as those formed from U.S. Pat. Nos. 6,855,774 and 6,878,778 and other FFKMs as well have been blended as described in U.S. Pat. No. 8,367,776 B2. That patent describes compositions with such polymers as well as with one or more additional FFKM, wherein two of the FFKM compounds in the composition differ in terms of their perfluoroalkyl vinyl ether (PAVE) monomer content by about 5 to about 25 mole percent. Such blends provide the ability to form compositions which can function well without the use of fluoroplastic fillers and are alternatives to and in some cases improvements over such filled materials. Such blends also do not require high temperature mixing, as is required in melt blended compositions, and provide crack-resistance in the presence of harsh chemicals, and good thermal and plasma resistant properties.
Thus, while various blends, unique cure systems and fillers have been proposed as noted above to improve FFKMs by forming clean, or unfilled compounds and/or by blending FFKMs with varied PAVE content as described in the patents and publications noted above, there continues to be a need in the art for further improvements to perfluoroelastomer compositions which, upon cure, retain good and lower compression set values, good plasma resistance, and good physical properties such as relatively low hardness, and sufficient strength and elongation, and can also demonstrate good stiction properties, so as to continue to meet the increasingly demanding requirements for use in high-end sealing applications like those of semiconductor processing, particularly where high temperature environments are encountered.